This invention relates to the field of image projectors. More particularly, this invention relates to the field of modulation of light out of the focal plan in a grating light valve based projection system.
In recent years, light modulators have been developed using MEMS (micro-electro-mechanical systems) technology in which moveable elements are configurable to direct light. An example of such light modulators is a grating light valve type device (GLV type device) taught in U.S. Pat. No. 5,311,360 to Bloom et al., in which the GLV type device is configurable in a reflecting mode and a diffracting mode. The GLV type device taught by Bloom et al. is isometrically illustrated in FIG. 1. The GLV type device 10 includes moveable elongated elements 12 suspended over a substrate 14.
A first side view of the GLV type device 10 of the prior art is illustrated in FIG. 2A, which shows the GLV type device 10 in the reflecting mode. The moveable elongated elements 12 each include a first reflective coating 16. Interspersed between the moveable elongated elements 12 are second reflective coatings 18. In the reflecting mode, upper surfaces of the first and second reflective coatings, 16 and 18, are separated by a height difference of a half wavelength xcex/2 of incident light I. The incident light I reflecting from the second reflecting coatings 18 travels a full wavelength further than the incident light I reflecting form the first reflecting coatings 16. So the incident light I, reflecting from the first and second reflecting coatings, 16 and 18, constructively combines to form reflected light R. Thus, in the reflecting mode the GLV type device 10 produces the reflected light R.
A second side view of the GLV type device 10 of the prior art is illustrated in FIG. 2B, which shows the GLV type device in the diffracting mode. To transition from the reflecting mode to the diffracting mode, an electrostatic potential between the moveable elongated elements 12 and the substrate 14 moves the moveable elongated elements 12 to contact the substrate 14. To maintain the diffracting mode, the electrostatic potential holds the moveable elongated elements 12 against the substrate 14. In the diffracting mode, the upper surfaces of the first and second reflective coatings, 16 and 18, are separated by a quarter wavelength xcex/4 of the incident light I. The incident light I reflecting from the second reflecting surfaces 18 travels a half wavelength further than the incident light I reflecting from the first reflective coatings 16. So the incident light I, reflecting from the first and second reflecting coatings, 16 and 18, destructively interferes to produce diffraction. The diffraction includes a plus one diffraction order D+1 and a minus one diffraction order Dxe2x88x921. Thus, in the diffracting mode, the GLV type device 10 produces the plus one and minus one diffraction orders, D+1 and Dxe2x88x921.
A first alternative GLV type device of the prior art is illustrated in FIGS. 3A and 3B. The first alternative GLV type device 10A includes first elongated elements 22 interdigitated with second elongated elements 23. The first elongated elements 22 include third reflective coatings 26; the second elongated elements 23 include fourth reflective coating 28. In the reflecting mode, illustrated in FIG. 3A, the third and fourth reflective coatings, 26 and 28, are maintained at the same height to produce the reflected light R. In the diffracting mode, illustrated in FIG. 3B, the first and second reflected coatings, 26 and 28, are separated by the second height difference of the quarter wavelength xcex/4 of the incident light Ito produce the diffraction including the plus one and minus one diffraction orders, D+1 and Dxe2x88x921.
A display system utilizing a GLV type device is taught in U.S. Pat. No. 5,982,553 to Bloom et al. The display system includes red, green, and blue lasers, a dichroic filter group, illumination optics, the GLV type device, Schlieren optics, projection optics, a scanning mirror, and display electronics, which project a color image onto a display screen. The red, green, and blue lasers, driven by the display electronics and coupled to the GLV type device (via the dichroic filter group and the illumination optics) sequentially illuminate the GLV type device with red, green, and blue illuminations. The GLV type device, driven by the display electronics, produces a linear array of pixels which changes with time in response to a signal from the display electronics, each pixel configured in the reflecting mode or the diffracting mode at a given instant in time. Thus, the GLV type device produces sequential linear arrays of red, green, and blue pixels with each of the red, green, and blue pixels in the reflecting mode or the diffracting mode.
The red, green, and blue pixels are then coupled to the Schlieren optics which blocks the reflecting mode and allows at least the plus one and minus one diffraction order, D+1 and Dxe2x88x921, to pass the Schlieren optics. Thus, after passing the Schlieren optics, the linear arrays of the red, green, and blue pixels have light pixels corresponding to the pixels at the GLV type device in the diffracting mode and dark pixels corresponding to pixels at the GLV type device in the reflecting mode. The projection optics (via the scanning mirror) project the linear arrays of the red, green, and blue pixels onto the display screen while the scanning mirror, driven by the display electronics, scans the linear arrays of the red, green, and blue pixels across the display screen. Thus, the display system produces a two dimensional color image on the display screen.
A graphical top cross sectional view of a GLV type device is illustrated in FIG. 4. A scale in millimeters is provided to show an approximate layout of the system. In a system such as this, the GLV type device 10 is positioned such that the focal point 36 of the incident light I is located on the surface of the GLV type device 10. A seal glass 34 is positioned over the GLV type device 10 forming an air gap 32 between the GLV type device 10 and the seal glass 34. In this prior art embodiment, the seal glass 34 permits the incident light I to pass to the air gap 32 and then to the GLV type device 10. The reflected light R travels through the air gap 32 and the seal glass 34 before a portion of the reflected light R is reflected by an outer surface 38 of the seal glass. As depicted in FIG. 4, this second reflected light R2 eventually leaves the seal glass at a different point than the reflected light R. This second reflected light R2, as well as additional reflected light derivative of R2, creates undesirable background light in the final display.
An alternative display system utilizing the GLV type device includes the red, green, and blue lasers, red, green, and blue illumination optics; first, second, and third GLV type devices; the dichroic filter group; the projection optics; the scanning mirror; and the display electronics. The red, green, and blue lasers, via the red, green, and blue illumination optics, illuminate the first, second, and third GLV type devices, respectively. The first, second, and third GLV type devices produce the linear arrays of the red, green, and blue pixels, respcetively, in response to signals from the display electronics. The dichroic filter group directs the light from the linear arrays of the red, green, and blue pixels to the Schlieren optics, which allows at least the plus one and minus one diffraction order, D+1 and Dxe2x88x921, to pass the Schlieren optics. The projection optics, via the scanning mirror, project the linear arrays of the red, green, and blue pixels onto the display screen while the scanning mirror, driven by the display electronics, scans the linear arrays of the red, green, and blue pixels across the display screen. Thus, the alternative display system produces the two dimensional color image on the display screen.
Examples of applications for a GLV type device based display system include a home entertainment system, a boardroom application, and a cinema application among others. In the home entertainment system or the boardroom application, the GLV type device based display system projects the two dimensional color image onto the display screen located on a wall. In the cinema application, the GLV type device based display system projects the two dimensional color image from a display booth onto a cinema screen.
A GLV type device based display system may also be utilized in printing applications. In such a case, the system would not include a scanning mirror, and the printing media, replacing a screen, would move to effectuate printing from a fixed line of light.
What is needed is a method of reducing the problems of space, heat, noise, and vibration in the home entertainment system, the boardroom application, and the cinema application. The problem of heat in various applications affects the operation of the GLV type device and shortens the GLV type device""s life span. Specifically, illumination intensity affects the operation of the GLV type device and ultimately shortens the GLV type device lifespan when a GLV type device is positioned at the focal plane of a line illumination. Typically, the optical focal point in a system such as this is at the GLV type device modulator, thereby exposing the GLV type device to high optical intensities anywhere from 50 Watts to 100 Watts per color. These intensities must be reduced in order to preserve the high quality function of the GLV type device over longer periods of time, thus extending the overall life span of the GLV type device. What is also needed are methods for further correcting imperfections and defects in the line illumination that cause imperfections in the video display, while implementing an additional filtering step to eliminate more background light, thus further clarifying the display.
The present invention is a display apparatus and method for modulating light out of the focal plan in a grating light valve type device based projection system. The display apparatus and method includes positioning a grating light valve type device near but not at a focal plane of a line illumination such that the grating light valve type device produces either a two dimensional real image and a two dimensional virtual image.
The grating light valve type device in the present invention is also configured such that the modular members of the grating light valve type device are neither parallel nor perpendicular with the boundaries of the effective pixel area to broaden the width of the video line, thus correcting imperfections and striations attributable to ribbon defects. A seal glass is coupled with the grating light valve type device in such a manner that the line illumination must pass through the seal glass and an air gap between the seal glass and the grating light valve type device before reaching the grating light valve type device.
The present invention also embodies an absorbing aperture that is affixed to the outer surface of the seal glass at the focal plane of the line illumination in order to filter additional background light to achieve higher contrast, thus providing a much clearer picture.